Feature/sns producer message attributes

[massa-man]

PR for issue #46

[YanivD]

@massa-man Thank you for opening a PR and making the effort. We'll review it in the following days

[YanivD]

Thanks again for your contribution!

Please implement few changes before we'll be able to merge and publish it:

1. This feature should be added also for sqs-producer.
2. Please document this feature in README file.
3. WDYT move snsMessageAttributes prop inside an options object? In order to make it easy in the future to add new options.

🙏

[massa-man]

Thanks for the comments. Happy to implement the changes proposed. As for your point 1 (add to sqs-producer), the message attributes for SNS are different from SQS message attributes. I can look at adding them here, but technically it is not the same functionality.

https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-message-metadata.html#sqs-message-attributes

https://docs.aws.amazon.com/sns/latest/dg/sns-message-attributes.html

[YanivD]

@massa-man thanks for pointing the difference between SNS message attributes and SQS message attributes.
Please ignore suggested point 1, and update the PR according point 2+3.